Field of the Invention
The present invention relates to a rotary damper for applying damping forces to the keyboard cover of a piano, for example, as it is opened and closed, and more particularly to a rotary damper free of backlash. Description of the Related Art:
One conventional rotary damper has a structure disclosed in Japanese laid-open patent publication No. 5-10366 as shown in FIG. 18 of the accompanying drawings. As shown in FIG. 18, when a movable shaft 2 rotates about its own axis in the direction indicated by the arrow a, a planar surface 9c of a valve 9a turns counterclockwise about a bearing recess 8a under the counterclockwise pressure from working oil 10 in a chamber A, bringing an arcuate surface 9d of the valve 9a into intimate contact with an inner wall surface 1k of a cylinder 1, so that the valve 9a is closed thereby to produce a high torque. When the movable shaft 2 rotates about its own axis in the direction indicated by the arrow b, the arcuate surface 9d of the valve 9a is subject to clockwise forces from the working oil 10 and turns clockwise about the bearing recess 8a. The arcuate surface 9d is spaced from the inner wall surface 1k, so that the valve 9a is opened thereby to produce a low torque.
When the movable shaft 2 is temporarily stopped while it is being rotated in the direction to open the valve 9a, and is then turned in the direction to close the valve 9a, it takes a certain period of time for the valve 9a spaced from the inner wall surface 1k to turn into intimate contact with the inner wall surface 1k again under the pressure of the working oil 10. Until the valve 9a is brought into intimate contact with the inner wall surface 1k, the rotary damper is unable to produce a high torque.
The idle period of time or interval during which the transition to a high torque is not available from the rotary damper is referred to as backlash. Because of the backlash, the conventional rotary damper fails to provide a quick damping effect which would be needed when damping forces are required to dampen a closing cover, for example. There has been a demand for a rotary damper which has a quick response to provide the transition to a high torque.